1. Technical Field
The present invention relates to a fixed type constant velocity universal joint, and more specifically, to a fixed type constant velocity universal joint that is used in a power transmission system for automobiles and various industrial machines and allows only angular displacement between two shafts on a driving side and a driven side.
2. Background Art
For example, a plunging type constant velocity universal joint that is capable of axial displacement while forming an operating angle including a relatively low maximum operating angle is assembled normally on an inboard side (differential side) of a front drive shaft of an automobile. On an outboard side (wheel side), a fixed type constant velocity universal joint that is capable of forming high operating angles but is not displaced in the axial direction is assembled in consideration of steering of the wheel.
FIG. 14 illustrate a Rzeppa type constant velocity universal joint 101 as an example of the fixed type constant velocity universal joint used on the outboard side. FIG. 14a is a vertical sectional view illustrating a state in which an operating angle is set to 0°, and FIG. 14b is a schematic view illustrating a state in which a maximum operating angle is formed. As illustrated in FIG. 14a, the constant velocity universal joint 101 includes, as main components, an outer joint member 102, an inner joint member 103, balls 104, and a cage 105. The outer joint member 102 has a spherical inner peripheral surface 106 including eight track grooves 107 formed equiangularly along an axial direction. The inner joint member 103 has a spherical outer peripheral surface 108 including track grooves 109 formed equiangularly along the axial direction so as to face the track grooves 107 of the outer joint member 102. Eight balls 104 for transmitting torque are interposed between the track grooves 107 of the outer joint member 102 and the track grooves 109 of the inner joint member 103. The cage 105 for holding the balls 104 is arranged between the spherical inner peripheral surface 106 of the outer joint member 102 and the spherical outer peripheral surface 108 of the inner joint member 103. An outer periphery of the outer joint member 102 and an outer periphery of a shaft coupled to the inner joint member 103 are covered with a boot, and grease as a lubricant is sealed inside the joint (none of which is shown).
As illustrated in FIG. 14a, the cage 105 has a spherical outer peripheral surface 112 fitted to the spherical inner peripheral surface 106 of the outer joint member 102, and a spherical inner peripheral surface 113 fitted to the spherical outer peripheral surface 108 of the inner joint member 103. The spherical outer peripheral surface 112 and the spherical inner peripheral surface 113 each have a curvature center formed at a joint center O. On the other hand, a curvature center Oo of a ball-raceway center line x of the track groove 107 of the outer joint member 102 and a curvature center Oi of a ball-raceway center line y of the track groove 109 of the inner joint member 103 are offset in the axial direction by equal distances with respect to the joint center O. With this, when the joint forms an operating angle, the balls 104 are always guided in a plane obtained by bisection of an angle formed between axial lines of the outer joint member 102 and the inner joint member 103. As a result, rotational torque is transmitted at a constant velocity between the two axes.
As illustrated in FIG. 14b, a maximum operating angle θmax, which may be set as a main function of the fixed type constant velocity universal joint 101, depends on an angle at which an inlet chamfer 110 provided at an opening rim of the outer joint member 102 and a shaft 111 interfere with each other. In order to secure permissible torque to be transmitted, the shaft 111 has an axial diameter d set on a joint-size basis. When a large inlet chamfer 110 is formed, a length of the track groove 107 of the outer joint member 102, on which the ball 104 is brought into abutment (hereinafter referred to as “effective track length”), is insufficient. As a result, the ball 104 drops off the track groove 107, and rotational torque cannot be transmitted. Thus, how the inlet chamfer 110 is formed while securing the effective track length of the outer joint member 102 is an important factor in securing the operating angle. In the Rzeppa type constant velocity universal joint 101, the curvature center Oo of the ball-raceway center line X of the track groove 107 of the outer joint member 102 is offset to an opening side, and hence there is an advantage in terms of the maximum operating angle. However, the maximum operating angle θmax is approximately 47°.
Further, in comparison with a conventional six-ball constant velocity universal joint, the Rzeppa type constant velocity universal joint 101 of an eight-ball type has a smaller track offset amount, a larger number of balls, and a smaller diameter. Thus, it is possible to provide a high-efficient constant velocity universal joint that is lightweight and compact, and suppresses torque loss. However, at the operating angle of 0°, wedge angles formed between the track grooves 107 and 109 of the outer joint member 102 and the inner joint member 103, which face each other, are opened toward the opening side of the outer joint member 102. Thus, due to forces applied in the axial direction from the track grooves 107 and 109 to the balls, loads to be applied to the spherical contact portions 106 and 112 of the outer joint member 102 and the cage 105 and the spherical contact portions 108 and 113 of the inner joint member 103 and the cage 105 are generated in a certain direction. Thus, this configuration leads to restriction on a further increase in efficiency and suppression of heat generation.
In order to achieve much higher efficiency and less heat generation than those can be achieved by the Rzeppa type constant velocity universal joint 101 of the eight-ball type described above, there has been proposed a fixed type constant velocity universal joint of a track groove crossing type (Patent Document 1). This constant velocity universal joint is illustrated in FIG. 15. FIG. 15a is a vertical sectional view illustrating a state in which the operating angle is set to 0°, and FIG. 15b is a schematic view illustrating a state in which a high operating angle is formed. As illustrated in FIG. 15a, a constant velocity universal joint 121 includes, as main components, an outer joint member 122, an inner joint member 123, balls 124, and a cage 125. The constant velocity universal joint 121 is a constant velocity universal joint of the track groove crossing type. Although not shown, planes each including the ball-raceway center line x of corresponding one of eight track grooves 127 of the outer joint member 122 are inclined with respect to a joint axial line n-n, and inclination directions of the planes defined in the track grooves 127 that are adjacent to each other in the circumferential direction are set opposite to each other. Further, track grooves 129 of the inner joint member 123 each have the ball-raceway center line y formed to be mirror-image symmetrical with the ball-raceway center line x of corresponding one of the paired track grooves 127 of the outer joint member 122 with respect to a plane P including the joint center O at the operating angle of 0°.
In the vertical cross-section illustrated in FIG. 15a, the track grooves 127 formed in a spherical inner peripheral surface 126 of the outer joint member 122 each extend in a circular-arc shape along the axial direction, and have a curvature center located at the joint center O. In a spherical outer peripheral surface 128 of the inner joint member 123, the track grooves 129 that face the track grooves 127 of the outer joint member 122 each extend in a circular-arc shape along the axial direction, and have a curvature center located at the joint center O. Eight balls 124 for transmitting torque are interposed in intersecting portions between the track grooves 127 of the outer joint member 122 and the track grooves 129 of the inner joint member 123. The cage 125 for holding the balls 124 is arranged between the spherical inner peripheral surface 126 of the outer joint member 122 and the spherical outer peripheral surface 128 of the inner joint member 123. The cage 125 has a spherical outer peripheral surface 132 fitted to the spherical inner peripheral surface 126 of the outer joint member 122, and a spherical inner peripheral surface 133 fitted to the spherical outer peripheral surface 128 of the inner joint member 123. The spherical outer peripheral surface 132 and the spherical inner peripheral surface 133 each have a curvature center formed at the joint center O. In the constant velocity universal joint 121, the curvature center of each of the ball-raceway center lines x and y of the respective track grooves 127 and 129 of the outer joint member 122 and the inner joint member 123 is not offset in the axial direction with respect to the joint center O. However, the track grooves 127 and 129 that are inclined and face each other intersect with each other, and the balls 124 are interposed in those intersecting portions. With this, when the joint forms an operating angle, the balls 124 are always guided in a plane obtained by bisection of an angle formed between axial lines of the outer joint member 122 and the inner joint member 123. As a result, rotational torque is transmitted at a constant velocity between the two axes.
In the fixed type constant velocity universal joint 121 of the track groove crossing type described above, the track grooves 127 and 129 of the outer joint member 122 and the inner joint member 123 that are adjacent to each other in the circumferential direction are inclined in the directions opposite to each other. Thus, forces in the opposite directions are applied from the balls 124 to pocket portions 125a adjacent to each other in the circumferential direction of the cage 125. Due to the forces in the opposite directions, the cage 125 is stabilized at a position of the joint center O. Therefore, a contact force between the spherical outer peripheral surface 132 of the cage 125 and the spherical inner peripheral surface 126 of the outer joint member 122, and a contact force between the spherical inner peripheral surface 133 of the cage 125 and the spherical outer peripheral surface 128 of the inner joint member 123 are suppressed. Thus, the joint is smoothly operated under high load and in high speed rotation, and torque loss and heat generation are suppressed. As a result, higher durability can be achieved.
The fixed type constant velocity universal joint 121 described above is excellent as a joint that suppresses heat generation. However, there is a problem as follows. As illustrated in FIG. 15b, when a large inlet chamfer 130 is formed in the outer joint member 122 in a structure in which the curvature center of the track groove 127 matches with the joint center O, the effective track length of the track groove 127 of the outer joint member 122 is insufficient. Thus, when a high operating angle θ is formed, the ball 124 drops off the track groove 127, with the result that higher operating angles may not be formed.